What is commonly known in the prior art for forming a color image or black-and-white image includes a technique (so-called a pigment thermal transfer method) wherein the ink sheet containing the thermally diffusive pigment subjected to diffusion and migration by heating is arranged face to face with an image receiving layer of the image receiving sheet, and the thermally diffusive pigment is thermally transferred onto this image receiving layer as an image, using a thermal printing means such as a thermal head or laser, whereby an image is formed. Such a thermal transfer method is highly evaluated as a technique of producing a high-quality image comparable to the image obtained from silver halide photography, using digital data, without the need of utilizing any processing solution such as a developing solution.
To provide a superb printing characteristic in the recording method based on thermal transfer technique using a pigment, it is important to incorporate a heat insulating function and cushioning function into the thermal transfer image receiving sheet. This importance has been recognized in the prior art.
One of the known methods for meeting this requirement is to bond an expanded film having both the heat insulating function and cushioning function onto a substrate, and to arrange an image receiving layer thereon. This method causes the expanded film to be shrunken by the heat produced by coating of the image receiving layer, with the result that a curl will be found in the final product. Such a problem has been observed in the prior art. To overcome these difficulties, a study has been made to develop a new functional layer having the heat insulating function and cushioning function. A study has also been made to provide a thermal transfer image receiving sheet and its manufacturing method without using the step of bonding a expanded film or the like, in order to eliminate the possibility of a curl being produced by heat history in the manufacturing process. Such a method is exemplified by a coating method.
For example, a thermal transfer image receiving sheet having an intermediate layer and image receiving layer provided on a substrate has been disclosed. This sheet is characterized in that the intermediate layer is composed of a layer mainly comprising:
hollow particles having a diameter of 0.1 through 100 μm obtained by thermal expansion of thermally expanding plastic substances; or
hollow polymer particles, shaped like a micro-capsule, having a diameter of 0.1 through 20 μm; and
high polymers impervious to organic solvent (Patent Document 1).
To get a heat insulating effect when using the hollow particles, it is generally necessary to ensure a high fill factor sufficient to allow use of the void between particles. It is not possible to avoid irregularities in surface smoothness or reduction in the strength of the layer resulting from a low percentage of binder. Not only that, when a further layer is coated thereon, air bubbles present in the voids between hollow particles tend to rise and to give an adverse effect to the image receiving layer in the final stage, with the result that uneven printing will occur.
A solution to these problems is disclosed in an image receiving sheet formed by sequential lamination of a heat insulating layer and an image receiving layer containing hollow particles on the substrate of cellulose paper, for example. In this sheet, the substrate has an air permeability is 1,000 sec. or more without exceeding 3,500 sec. (Patent Document 2). However, this proposal is based on the concept of ensuring that air bubbles present in the void between hollow particles are released toward the substrate, wherein presence of these air bubbles raises a problem when a layer is coated on the layer containing hollow particles. This proposal makes it difficult for air bubbles to rise, but tends to encourage permeation of the liquid coated on the layer to be arranged on the layer containing hollow particles. This method allows film thickness to become uneven. Not only that, this approach causes the compositions of the coated liquid to fill the void between hollow particles close to the surface of the heat insulating layer where the heat insulating function is most effective. Thus, the originally intended heat insulating effect cannot be utilized. This method has such a problem.
Against this backdrop, there has been a long felt need for a thermal transfer image receiving sheet or manufacturing method thereof, capable of making the maximum use of the advantages of the heat insulating layer of the hollow particles, providing high-density printing characteristics, and ensuring uniform printing.
Further, a method for manufacturing a thermal transfer image receiving sheet is disclosed, wherein the water-based intermediate layer containing hollow particles, and the water-based image receiving layer containing a mold releasing agent adjacent thereto are coated according to the wet-on-wet method (Patent Document 3). This Patent Document 3 discloses a process of improving the viscosity of each coating solution and controlling the excessive mixture of liquids. When exposed to a high temperature in the step of drying, not a small amount of substances transfer takes place on the two adjacent liquid interfaces of high fluidity. For example, if the mold releasing layer in the wet image receiving layer transfers in the wet heat insulating layer, insufficient bondage occurs between the hollow particles close to the surface of the heat insulating layer. Smoothness of the interface will be reduced, with the result that uneven printing will occur.
[Patent Document 1] the Official Gazette of Japanese Patent Tokkaihei 11-321128
[Patent Document 2] the Official Gazette of Japanese Patent Tokkai 2004-9572
[Patent Document 3] the Official Gazette of Japanese Patent Tokkaihei 6-171240